A display for displaying images is one of light emitting devices indispensable in the modern life and takes various forms such as a so-called TV monitor, a liquid crystal display developed rapidly in recent years, an organic EL display expected to be developed, and the like depending on the usage. Among the above, the organic EL display is best noted as a next generation flat panel display device.
A light emission mechanism of an electroluminescent device constituting the organic EL display is such that a luminescent layer made from a luminescent composition (hereinafter referred to as thin film) is provided between electrodes so that electrons injected from a cathode are recombined with holes injected from an anode at the recombination center of the luminescent layer to form molecular excitons when a current is supplied and photons discharged when the molecular excitons return to the ground state are used for the light emission.
As kinds of the molecular exciton that the luminescent composition forms, there are a singlet exciton and a triplet exciton. However both excitons are included in the specification.
In such an EL display element (hereinafter referred to as EL element), the thin film is usually formed with such a thin film thickness as less than 1 μm. Furthermore there is no need of a backlight that is used in a conventional liquid crystal display since the EL element is a self-emitting element in which a luminescent film itself emits light. Therefore, it is a great advantage to be capable of manufacturing the EL element which is very thin and lightwheight.
Furthermore, in the case of a thin film having a thickness of substantially 100 to 200 nm, a time from injection of a carrier to recombination thereof is substantially tens nanoseconds in consideration of the carrier mobility of the luminant film. Consequently, even when a process from the recombination of the carrier to light emission is included, the light emission is caused within several microseconds. Accordingly, it is also advantageous in that a response speed is very fast.
Still furthermore, since the EL element is a light-emitting element of carrier-injection type, the direct current voltage drive is possible and it is hard to generate a noise. In addition, when a uniform and very thin film having a thickness of substantially 100 nm is made of an appropriate material, drive at a voltage of several volts can be realized. That is, the EL element is not only excellent in the visibility because it is self-emitting and large in a viewing angle but also has characteristics such as being thin and lightwheight, high in the response speed, and drivable at direct current and low voltage. Accordingly, it is expected as a next generation light-emitting element.
As mentioned above, a light emission mechanism of an electroluminescent device constituting the organic EL display is such that a luminescent layer made from a luminescent composition is provided between electrodes so that electrons injected from a cathode are recombined with holes injected from an anode at the recombination center of the luminescent layer to form molecular excitons when a current is supplied and photons discharged when the molecular excitons return to the ground state are used for the light emission. Accordingly, one of preconditions for manufacturing a light emitting device of good efficiency is to inject the holes and the electrons efficiently into the thin film.
Under typical electroluminescent device operation conditions, a current of about 100 mA/cm2 is injected into the organic thin film inherently having a high electrical resistance. In order to realize such high density current injection, it is necessary to keep the sizes of a barrier against the holes injected from the anode and a barrier against the electrons injected from the cathode as small as possible. That is to say, it is necessary to use a metal having a small work function for the cathode and to select an electrode having a large work function for the anode. By selecting various metals and alloys for the cathode, it is practically possible to control the work function at will. In contrast, since transparency is required of the anode in general electroluminescent devices, the material to be used for the anode is limited to transparent conductive oxides under the current situation, and there is no alternative but to select some oxide conductive films such as an indium-tin oxide (hereinafter abbreviated to ITO) film in view of stability, transparency, resistivity, and the like at present.
The ITO electrode is indium doped tin which enter a substitution position of indium. Tins and slight amount of oxygen defects become donors to partially fill a conduction band, and thereby the conductivity appears. The ITO is deposited on glass by means of a sputtering method, an ion beam sputtering method and a vapor phase growth method. An electrode which is highly transparent and low in the resistance can be prepared by selecting a dope amount of tin appropriately.
However, since a surface of the ITO is not necessarily flat, therefore it is pointed out that a contact with a thin film that is used in an EL element deteriorates, or pinholes are caused in the thin film. This is said one of reasons that deteriorate an EL element. Furthermore, a work function of the ITO film can be changed to a certain degree by a process of film formation and a surface treatment, but such means have its limit. Therefore it is hard to reduce the hole injection barrier.
As one of methods to reduce the barrier against hole injection from the ITO cathode, an insertion of a buffer layer on the ITO film is known. By optimizing an ionization potential of the buffer layer, it is possible to reduce the hole injection barrier. The above-described buffer layer is called a hole injection layer. Materials which can function as the hole injection layer are generally classified into metal oxides, low molecular organic compounds, and high molecular compounds. Examples of the metal oxides are vanadium oxide, molybdenum oxide, ruthenium oxide, aluminum oxide, and the like. Examples of the low molecular organic compounds are starburst amines such as m-MTDATA, metal phthalocyanine, and the like. As the high molecular compounds materials, conjugate polymers such as polyaniline and a polythiophene derivative are known.
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By using the above-described materials for the hole injection layer, the hole injection barrier is reduced and the holes are efficiently injected, thereby improving the efficiency and life of the EL device and reducing a driving voltage. Polymer base materials is characterized in that it can be coated on a surface of the ITO by means of such as a spin coat method or an ink jet method. In particular, the ink jet method has a benefit that it is capable of forming an arbitrary microscopic pattern by controlling a position where a liquid drop sticks to a substrate and is low cost and convenient, so plays an important role in a manufacturing technology of EL elements.
Thus, polymer base hole injecting materials capable of applying a low cost and convenient ink jet method are very hopeful materials; however, there are problems described below. The conjugate polymers such as abovementioned polyaniline and polythiophene hardly exhibit by itself the conductivity. When these are mixed with a strong acid such as camphor sulfonic acid or poly(styrene sulfonic acid), that is, when strong acid is doped to the conjugate polymer, high conductivity is developed. Thus doped conductive conjugate polymer works as a hole injecting material; however, since strong acid is used as a doping agent (hereinafter, referred to as “dopant”), a thin film and the ITO that come into contact with the hole injecting layer are likely to be largely damaged. Furthermore, in the case of an EL element being applied in a TV monitor and so on, an active matrix type light-emitting device on which thin film transistors (hereinafter, referred to as “TFT”) are mounted is adopted; however, when a hole injecting material containing the strong acid is deposited on a substrate on which the TFTs are mounted, the TFT characteristics are largely adversely affected.
Furthermore, the polymer base material that has been conventionally used as the hole injecting material is insoluble in an organic solvent and is supplied as a water suspension after sulfonic acid that is strong acid is doped. Accordingly, in the case of deposition on a water-repellent substrate by ink jet method or spin coat method, the suspension liquid does not disperse uniformly, therefore it is difficult to form a uniform thin film because thickness of the hole injecting layer come to uneven. In particular, it is not necessarily easy to form a uniform and thin film on a substrate thereon TFTs are mounted since insulating materials that are used to insulate between the respective pixels, in many cases, are oleophilic, that is, water-repellent.
In other words, the polymer base hole injecting materials are advantageous in that these can be applied in the ink jet method or the spin coat method that is low in cost and convenient and can improve characteristics of an EL element. However, the polymer base hole injecting materials that have been conventionally adopted have large problems such as mentioned above fundamentally and leave a room of improvement.